In a litte known sport called "orienteering", contestants are provided with a compass and a map marked with a course, the map also having North-South reference lines. The maps have a course laid out in segments touching on a number of "control" points at which there is a stamp which must be stamped on their map when they get to that point to prove they have been there. Also on the map are topographical markings and symbols indicating the terrain, whether it is wooded or barren, where streams and marshes are, etc.
Therefore, the orienteer must carefully analyze the map quickly before he leaves each point to get to the next to identify what appears on the map to be the easiest course. The most direct line almost certainly is not the easiest course, and might involve stumbling through brush and over rocky outcroppings which could have been avoided by following a contour line which essentially connects the two points.
Although not as popular in the United States as in other places, orienteering is extremely popular in Scandinavia and has produced participants who have followed these courses since early childhood and who are extremely capable.
The technique generally used currently by orienteers requires a compass which has an overlying rotatively adjustable direction line. The compass is put on the map with North to North, and the direction line is adjusted rotatively to align with the direction of the first segment of the multi-segmented course. The orienteer, after figuring roughly how he intends to follow the course, springs off onto the course, checking his direction occasionally by aligning his compass with North and looking at the direction line he has set for that particular segment of the course.
Once he reaches his control point, he does the same thing with the next segment, that is, puts his compass on the map with the North on the compass aligned with the map's North (this, of course, is independant of the direction the needle of the compass is pointing), and rotates the direction indicator on his compass parallel with the direction of the next segment of the course. Once again he springs off, checking his direction time to time by aligning the compass needle with magnetic North by the compass needle and glancing at his directional line on the compass, to see if his path approximates, or deviates from the requisite course indicated by the direction line on the map.
This requires two pieces of equipment, the compass and the map. When it is raining and when it is very cold, it sometimes becomes difficult to change the direction line on the compass, since this usually requires barehanded operation. Also, having two objects to carry through underbrush and trees increases the chances of losing one and reduces one's manuverability slightly.
It would, therefore, be desireable to eliminate one of these items.